1. Field of the Invention
The present invention relates to a novel vacuum circuit breaker, a vacuum valve (or vacuum switch) used in the same, an electric contact used in the vacuum valve, and a method For making the electric contact.
2. Description of the Prior Art
An electrode structure in a vacuum circuit breaker comprises a pair of fixed electrode and movable electrode. The fixed and movable electrodes each comprise an arc electrode, an arc electrode support member for supporting the arc electrode, a coil electrode contiguous to the arc electrode support member, and an electrode rod provided at an end portion of the coil electrode.
The arc electrode is exposed to arc directly for breaking a high voltage and a large current flow. In view of this point, the arc electrode is required to satisfy the basic conditions of large breaking capacity, high withstand voltage value, small contact resistance value (high electrical conductivity), high fusion resistance, little contact erosion and small chopped current value. However, it is difficult to satisfy all of these characteristics, so in general there is used an arc electrode material which satisfies particularly important characteristics according to for what purpose it is to be used, while somewhat sacrificing the other characteristics. As an example of a method for producing an arc electrode material for breaking high voltage and large current, a method of infiltrating Cu into Cr or Cr--Cu skeleton is disclosed in Japanese Patent Laid Open No. 96204/88. Further, a similar method is disclosed in Japanese Patent Publication No. 21670/75.
On the other hand, the arc electrode support member not only serves as a reinforcing member for the arc electrode but also exhibits the effect of generating a vertical magnetic field by adopting a suitable shape thereof. And as the material of the arc electrode support member there is used pure Cu which is superior in conductivity.
The coil electrode also serves as a reinforcing member for the arc electrode and the arc electrode support member, as disclosed in Japanese Patent Publication No. 17335/91, but its main functions are to make the arc electrode generate a vertical magnetic field which is attained by adopting a suitable shape of the coil electrode, allowing arc generated at the arc electrode to be diffused throughout the entire arc electrode, to effect forced cut-off. The material of the coil electrode is pure Cu like that of the arc electrode support member.
The electrode comprising such arc electrode, arc electrode support member, coil electrode and electrode rod is fabricated through the steps of production and machining of the arc electrode material, machining of the arc electrode support member, coil electrode material and electrode rod, as well as assembly and soldering of the components.
The arc electrode is fabricated in the following manner. First, an arc electrode material is produced by a so-called infiltration method wherein the powder of Cr, Cu, W, Co, Mo, W, V or Nb, or of an alloy thereof, is formed into a predetermined shape having predetermined composition and porosity, sintered, and thereafter molten Cu or alloy is infiltrated into the skeleton of the sitter, or by a so-called powder metallurgy method wherein the density is adjusted to 100% in the sintering step prior to the infiltration step. The arc electrode material thus produced is then formed into a predetermined shape by machining.
The arc electrode support member, coil electrode and electrode rod are each formed by cutting into a predetermined shape which facilitates generation of a vertical magnetic field from pure Cu.
The components which have thus been subjected to infiltration and subsequent machining are then assembled and thereafter soldered to give an electrode structure comprising a series of electrodes. According to the soldering method, a bonding material and a solder superior in wettability are inserted between adjacent ones of the arc electrode, arc electrode support member, coil electrode and electrode rod, and the temperature is raised in vacuum or in a reducing atmosphere to effect soldering. In this soldering method, however, considerable labor and time are required for alignment of the components at the time of their assembly for soldering, in addition to the labor and time required for machining, and a defect of soldering causes an accident such as breakage or drop-out of the electrodes. The electrode structure obtained by such a conventional method is inferior in all of uniformity, reliability and safety of electrode characteristics.
Recently, attempts to cut off high voltage and large current from the angle of design specifications of vacuum circuit breakers have been made. As an example, an improvement of the breaking performance has been made by increasing the breaking speed. As a result, however, the contact force between arc electrodes increases and an impulsive stress is imposed on the whole electrode structure at the time of opening or closing the electrodes, thus causing deformation of the electrodes with the lapse of time. Generally, an arc electrode material of high strength superior in breaking characteristic or fusion resistance is used as the arc electrode material, while pure Cu is used as the material of arc electrode support member, coil electrode and electrode rod. The yield strength of pure Cu is very low, and grooving is applied to a cross section for the purpose of creating a vertical magnetic field as mentioned above, so that there will occur deformation of the electrodes with the lapse of time because of being unbearable particularly against an impulsive stress. Such deformation of the electrodes causes inconvenience in the electrode opening/closing operation, fusion of the arc electrode, breakage or drop-out of the arc electrode, which may obstruct the opening/closing motion in an emergency.